The fabrication of complementary bipolar (CB) transistors on a common substrate is of great interest in the production of both analog and digital circuits. Digital circuits based on CB technology offer the possibility of high-speed performance coupled with little dissipation of standby power. Analog circuits based on CB technology offer high-speed, substrate isolated PNP transistors, capacitor structures, low and high sheet resistance resistors, and the ability to provide precision matching of components.
It has been a continuing technological challenge to integrate vertical PNP devices into a high performance NPN process without significantly degrading the performance of the NPN device. This is essential for high performance, low noise, and high frequency applications.
U.S. Pat. No. 4,951,115 issued Aug. 21, 1990, and U.S. Pat. No. 4,997,776 issued Mar. 5, 1991, naming Harame et al. as inventors, disclose a complementary bipolar transistor structure and a method of manufacturing the same. The transistor structure includes a vertical NPN transistor operating in the upward direction and a vertical PNP transistor operating in the downward direction. A stacked N-type/P-type base layer is used to form the base regions for both the NPN and PNP devices. This causes the base-collector junction of both transistors to have the same vertical profile.
The complementary bipolar transistor manufacturing method described in the '115 and '776 patents produces vertical NPN and PNP devices having closer performance characteristics than many other methods discussed in the art. However, the resulting devices still differ in some respects, and thus are not truly complementary. For example, the NPN and PNP transistors differ in structure and mode of operation (current flow direction). Furthermore, the performance of the NPN device is compromised relative to the PNP device because the NPN base/emitter area is much larger than the PNP device base/emitter area.
A common problem to many of the complementary transistor fabrication methods discussed in the art is that the mode of operation of the two devices is different, i.e., one operates in an upward direction and the other operates in a downward direction. This is a result of the order in which the material layers which form the emitter, base, and collector regions for the two devices are deposited and implanted with dopants. This asymmetry in the operation of the two devices, combined with the difference in transistor structure, produces a difference in the performance of the two devices.
Another problem inherent to most methods of fabricating complementary bipolar devices is that the methods lack the ability to independently control the characteristics of the transistors. For example, because the emitter of one transistor is formed from the same layer as the collector of the other transistor, it is not possible to independently optimize the parameters of the two regions to obtain the highest and most closely matched performance for the two devices. This leads to a compromising of the performance of the NPN transistor.
What is desired is a method for manufacturing truly complementary bipolar transistors on a common substrate. It is desired that the two devices have an identical structure and mode of operation, and that the performance of the two devices be closely matched so that they may be used for high performance, low noise, and high frequency applications. It is also desired that the manufacturing process flow be such that the characteristics of the two transistors can be independently controlled and optimized. These and other advantages of the present invention will be apparent to those skilled in the art upon a reading of the Detailed Description of the Invention together with the drawings.